How a French Drain in Your Basement Will Keep It Dry

Posted by Matthew Stock on Tuesday, September 22, 2015.

Many homeowners think that the way to keep their basement dry is to plug up holes and seal things, which has resulted in products like hydraulic cement, waterproofing paint and various sealants, some even in aerosol cans, being used with little success.

There certainly is a value to sealing components of a basement, particularly masonry walls and wall cracks, but the right processes and materials for doing so are none of the above.

Actually, one of the best ways to keep a basement dry has nothing to with sealing or coating or plugging anything. In fact, it keeps the basement dry by preventing the water from developing the pressure it takes to enter the basement in the first place.

A French drain, often called “drain tile,” installed inside or outside the foundation, will keep most basements dry and never requires maintenance or replacement.

A French Drain Keeps Your Basement Dry

French drains originated as a way to drain farmland and are generally considered to be an American innovation, named after the man who popularized their use, not after a country of origin. They were first constructed of terra cotta roofing tiles set one on top of the other to form a crude pipe that allowed water infiltration from the sides. This is why the system is known commonly today as “drain tile.”

When used in residential construction, the French drain sits alongside the footings of a foundation and creates a space to alleviate the pressure created by water underground. The water drains into the pipe instead and is carried off to a sump pump.

An interior installation is recommended when hydrostatic pressure under the foundation forces water into the basement through the cove joint between wall and floor or through cracks in the floor. This pressure is created by a rise in the water table, usually caused by heavy rains.

An interior French drain is installed by removing a strip of concrete floor around the perimeter and digging a shallow trench to the base of the footings. A bed of washed gravel is poured and perforated, corrugated plastic pipe, wrapped in a filter “sock,” is laid on top and connected to a sump basin. More gravel covers the pipe and the concrete floor is replaced.

A French drain can also be installed outside the home when oversaturated soil around the foundation creates pressure that causes seepage through mortar joints or porous masonry like concrete blocks. Most often this exterior French drain is added at the same an exterior waterproofing membrane is applied to the foundation walls.

Installing an exterior French drain is similar to the interior process. The bottom of the excavation made to apply the waterproofing membrane is filled with washed gravel and, in this case, perforated, rigid PVC pipe is installed on top and connected to a sump basin.

PVC is used in exterior installations because of its strength, which makes it more resistant to soil pressure. Once installed, it is covered with more gravel and the excavation is backfilled.

In either installation, the system is completed by installing a sump pump with adequate capacity to move water through a discharge pipe at a rate faster than the basin can fill. Sump pump discharges can lead to dry wells or to daylight or, when local ordinances permit, to direct connections with storm sewer systems.

Regardless of whether keeping your basement dry requires a French drain to be installed on the interior or the exterior, one fact remains – only a qualified basement waterproofing contractor has the knowledge and capability to install it properly. At U.S. Waterproofing, we have installed literally miles of French drains on both sides of the foundation wall on our way to becoming one of the country’s oldest and largest basement waterproofing contractors. Why not ask for our free advice?

Want to know more about how a French drain can keep your basement dry? Please post your questions in the Comments box below.